Polar arrows are vectors pointing from a less to the more electronegative atom and thus indicating that electrons are pushed towards the more electronegative atom.
Here, we have different hydrogen-halogen compounds. To rank the compounds in order of the increasing ionic character, we have to consider the EN of halogen atoms. In each case, the polar arrow will be directed towards the halogen atom as a more electronegative atom. Recall that EN decreases down the group as the atomic size increases. So, Cl is more electronegative than Br, which in turn is more electronegative than I. Therefore, the same order is followed for the bond polarity:
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